Documentation

Initilization Vectors for BlockCipher implementations (IV k) are
used for various modes and guarrenteed to be blockSize bits long.
The common ways to obtain an IV are to generate one (getIV or
getIVIO) or to use one provided with the ciphertext (using the
Serialize instance of IV).

zeroIV also exists and is of particular use for starting ctr
mode with a fresh key.